This invention relates to the separation of air, particularly to produce an oxygen product.
The separation of air by rectification at cryogenic temperatures to produce a gaseous oxygen product is a well known commercial process. As commonly practised, the process includes purifying compressed air to remove constituents such as carbon dioxide and water vapour of relatively low volatility in comparison with that of oxygen or nitrogen. The air is then cooled in a heat exchanger to about its saturation temperature at the prevailing pressure. The resulting cooled air is introduced into the higher pressure stage of a double rectification column comprising higher pressure and lower pressure stages. Both stages contain liquid-contact vapour means which enable there to take place intimate contact and hence mass exchange between a descending liquid phase and an ascending vapour phase. The lower and higher pressure stages of the double rectification column are linked by a condenser-reboiler in which nitrogen vapour at the top of the higher pressure stage is condensed by boiling liquid oxygen at the bottom of the lower pressure stage. The higher pressure stage provides an oxygen-enriched liquid feed for the lower pressure stage and liquid nitrogen reflux for that stage. The lower pressure stage produces an oxygen product and typically a nitrogen product. Usually, nitrogen product is taken from the top of the low pressure stage, and a waste nitrogen stream is withdrawn from a level a little bit below that at which the nitrogen gas is at its maximum purity level. The oxygen and nitrogen product streams and the waste nitrogen stream are returned through the heat exchanger countercurrently to the incoming compressed air stream and are thus warmed as the compressed air stream is cooled.
If desired, the process may also be used to produce an impure argon product. If such a product is desired, a stream of oxygen vapour enriched in argon is withdrawn from an intermediate level of the lower pressure stage and is fractionated in a third rectification column containing liquid-vapour contact means. This column is provided with a condenser at its top and some of the oxygen-enriched liquid withdrawn from the higher pressure stage may be used to provide cooling for this condenser. An argon product may be withdrawn from the top of the argon separation column and liquid oxygen may be returned from the bottom of the argon column to the lower pressure stage of the double rectification column.
Since the rectification of the air takes place at cryogenic temperatures, it is necessary to provide refrigeration for the process. This is conventionally done by taking a portion of the condensed air stream at a suitably low temperature and expanding it with the performance of external work in a turbine and then introducing it into either the higher pressure or lower pressure stage of the double rectification column. Sometimes, particularly if a proportion of the oxygen production is to be in the liquid phase, the compressed air stream is split and a minor portion of it is further compressed, cooled in the heat exchanger and then expanded in the turbine and introduced into the lower pressure stage of the rectification column. See, for example, U.S. Pat. No. 4,746,343 and DE-B-2854508. An alternative well known method of providing refrigeration is to take a nitrogen vapour stream from the higher pressure stage of the double rectification column to return the stream for part of the way through the heat exchanger and then to expand it with the performance of external work in a turbine which returns the nitrogen to a lower pressure nitrogen stream entering the cold end of the heat exchanger. Such cycles are described as prior art in EP-A-321 163 and EP-A-341 854.
Generally, therefore, in the production of oxygen gas product by cryogenic rectification of air, a single turbine is used to provide the refrigeration for the process. It has however been proposed to use more than one turbine to produce the necessary refrigeration when producing an oxygen product. First, if the oxygen product is required entirely in the liquid state, it has been proposed to use two separate turbines. The use of two such turbines in these circumstances is hardly surprising as the requirement to produce all the oxygen in the liquid state adds considerably to the overall requirement of the process for refrigeration. In GB-A-1 520 103 a first expander 17 produces a stream of cold air at -136.degree. F. (180.degree. K.) and a second expander 22 takes air at a temperature of -159.degree. F. (161.degree. K.) and by expansion reduces its temperature to -271.degree. F. (105.degree. K.), which air is then introduced into the higher pressure stage of the rectification column. A similar process is disclosed in U.S. Pat. No. 4,883,518. It has also been proposed to improve an air separation cycle in which the main refrigeration is provided by a first air turbine which does not supply air directly to the lower pressure stage of the rectification column by adding a second turbine that does just that. See for example EP-A-260 002. Such an expedient, however, requires both turbines to have an exit temperature of less than 110.degree. K.
In designing an air separation process, the conditions in the lower pressure stage of the double column are particularly important. Typically, it is desired to produce the product gases from the lower pressure stage at atmospheric pressure. In order to ensure that there is an adequate pressure for the products to flow through the heat exchange system it is desirable for the pressure at the top of the lower pressure stage of the double column to be fractionally above atmospheric pressure. The pressure at the bottom of the lower stage of the column will then depend on the number of theoretical stages of separation selected for the lower pressure column and the pressure drop per theoretical stage. Since it is typically necessary for the gaseous nitrogen at the top of the higher pressure stage to be about 2.degree. K. higher in temperature than the liquid oxygen at the bottom of the lower pressure stage for the condenser-reboiler to operate properly, the pressure at the bottom of the lower stage effectively determines the pressure at the top of the higher pressure stage of the double column. The pressure at the bottom of the higher pressure stage of the double column will thus depend on the value at the top of the stage, the number of theoretical stages of separation in the higher pressure stage of the double column, and the pressure drop per theoretical stage. The pressure at the bottom of the higher pressure column in turn dictates the pressure to which the incoming air needs to be compressed. Generally, at least in the lower pressure stage of the double column, the average pressure drop per theoretical liquid-vapour contact tray is normally above 500 Pa (0.075 psi). It is well known in the art that column packings may be used instead of distillation trays in order to effect liquid-vapour contact. One feature of such packings is that they tend to have lower pressure drops per theoretical stage of the separation than trays, although there is a tendency in modern tray design for air separation columns to reduce the pressure drop per theoretical tray below levels that have been traditionally used. Since the lower pressure stage may contain a large number of theoretical stages of separation (typically over 50 stages) designing the lower pressure stage with a low pressure liquid-vapour contact means, be it a packing or a multiplicity of trays, does have an appreciable influence on the operating parameters of the air separation cycle, and particularly makes possible a reduction in the pressure to which the incoming air needs to be compressed. Even though the total reduction in the pressure to which the incoming air may be compressed is typically in the order of 0.5 to 1 bar, we have surprisingly found that this pressure drop has a profound effect on the thermodynamic efficiency of the heat exchange system within the process and makes desirable substantial changes to the refrigeration system employed. Notwithstanding the fact that EP-A-321 163 and EP-A-341 854 both disclose the use of low pressure drop liquid-vapour contact means in the lower pressure stage of the distillation column, the refrigeration cycle that they employ in association with the double column is of a substantially conventional nature with just one turbine being used to expand a returning nitrogen stream from the higher pressure column to the pressure of the lower pressure column.